To examine the effect of various carbohydrates on the metabolic and hormonal response to exercise, 75 g glucose, fructose, or placebo were given to nine well-trained males (VO2 max 60 +/- 1 ml . kg-1 . min-1) 45 min before cycle ergometer exercise performed at 75% VO2 max for 30 min. After glucose ingestion, the rise in plasma glucose was 3-fold (P less than 0.005) in plasma insulin 2.5-fold (P less than 0.01) greater than after fructose. During exercise, after glucose administration plasma glucose fell from 5.3 +/- 0.3 to 2.5 +/- 0.2 mmol/l (P less than 0.001) and after fructose from 4.5 +/- 0.1 to 3.9 +/- 0.3 mmol/l (P less than 0.05). The fall in plasma glucose was closely related to the preexercise levels of plasma insulin (r = 0.82, P less than 0.001) and glucose (r = 0.81, P less than 0.001). Both glucose and fructose ingestion decreased the FFA levels by 40-50% (P less than 0.005) and during exercise they remained 30-40% lower after carbohydrate than placebo administration (P less than 0.02). This study suggests that glucose ingestion prior to exercise results in hypoglycemia during vigorous exercise, this rapid fall in plasma glucose is mediated, at least in part, by hyperinsulinemia, and fructose ingestion is associated with a modest rise in plasma insulin and does not result in hypoglycemia during exercise.